13.2 Scope ofStudy
4.1.3 Decomposition Kinetics Using TGA
The TGA kinetics approach is based on the Flynn & Wall method which requires at least three determinations at different linear heating rates, where in this case, 10°C/min, 15°C/min and 20°C/min are conducted. Figure 4.4 displayed are the TGA result
generated on the 'rambai' leave at an applied heating rate of 10°C/min. The plot shows
the percent mass a function of sample temperature. A single well-defined weight loss event is obtained with the onset temperature of 258.004 °C. As the heating rate is increased, the onset of decomposition is pushed to lower temperatures, reflecting the time, temperature dependency of the decomposition reaction. Similarly, as the heating rate is decreased, the onset temperature is moved to increasingly higher temperature.This is shown in Figure 4.5 for the 'rambai' leave at heating rates of 10, 15 and 20°C/min. Refer to Appendix III for the other single graphs/curves generated by TGA.
400 500
Figure 4.4: Rambai Leaves Decomposition at a Heating Rate of 10°C/min
30 130 230 330 430 530 Temperature (°C)
630
Figure 4.5: TGA Kinetics The approach assumes the basic Arrhenius equation [8],
(do/dt) = A exp (-Ea/RT)(l-a)n
730
-Rate ITC/mirt -Rate-BTJ/irtn Rate2ffC/min
830
(3)
Where a is the fraction composition, t is the time(s), (da/dt) is the rate of mass loss, Z is
the pre-exponential factor (1/second), Ea is the activation energy (J/mole, R is the gas constant (8.314 J/mole K) and n is the reaction order (dimensionless). Under the application of a constant heating rate, 0, and assuming a first order reaction (n=l), the
rate expression becomes:
do/(l- a) = (A/0)exp(-Ea/RT)dT (4)
As observed in Figure 4.5, a constant decomposition level is selected and the
corresponding temperature is determined for each different heating rate. In this case 10, 15 and 20% decomposition level is used. Refer to Table 4.3 for the corresponding temperature value. The measured values of temperature and TGA heating rate are then
used to calculatethe activationenergy by plotting log &vs. 1/T (Figure 4.6).
Table 4.3: Corresponding Temperature for each DifferentHeating Rates at Different Constant Decomposition Level
Decomposition
Level
Heating Rates, 0 fC/min)
Temperature,
T(°C) logo 1/T
10%
10 150 1.00 0.00667
15 110 1.18 0.00909
20 90 1.30 0.01111
15%
10 260 1.00 0.00385
15 240 1.18 0.00417
20 230 1.30 0.00435
20%
10 290 1,00 0.00345
15 280 1.18 0.00357
20 275 1.30 0.00364
Figure 4.6: log 0 vs. 1/T
The activation energy is assessed to be 67.7, 593.8 and 1577.5kJ/mole at the 10, 15 and 20% conversion level. The change in the decomposition kinetics is due to factors such
as anti-oxidants.
4.2 Adsorption Equilibrium Study
The effect of agitation time on the extraction/adsorption of metal ions is considered to
be of significant importance to study the extracting behavior of sorbent. Figure 4.7 shows equilibrium adsorption times of 60 ppb Hg (II) and 20 ppm Cd (II) with pH of 4,
volume of 5 mL, and adsorbent dosage of 0.1 g and conducted at room temperature (24- 25°C).3 4
Time (hrs)
—•—Hg 60 ppb .-^.-Cd 20 ppm
Figure 4.7: Extraction of Hg (II) and Cd (II)vs. Time (hrs)
It is observed that for both solution of Cd (II) and Hg (II), it is evident that adsorption
which was very rapid initially become slower with lapse of time. For Hg (II), 81.65% of
total extraction took place after 1 hour, while 53% extraction for Cd (II) in the first hour.At the initial stage, the process of adsorption was fast due to the availability of abundant free sites near the surface where the approach of the metal ions faced less hindrance.
With the lapse of time and decrease in the availability of active sites near the surface, the metal ions had to diffuse into the pores of the adsorbent ('rambai' leave) through the
interconnected pores and channels. It can be safely considered that the adsorption
process reaches equilibrium after 5 hours as observed in Figure 4.7, where the optimum
adsorptions are 96.67% and 90% for Hg (II) and Cd (II) respectively. In terms of amount of metal ions adsorbed per unit mass of adsorbent, Qe, for 60 ppb of Hg (II), 28.94ug/g is adsorbed while 4.955 mg/g amount of Cd (II) ions removed with imtial concentration of 20 ppm. Refer to Appendix III for the results calculated in terms of amount of metal ions adsorbed per unit mass of adsorbent.
43 Effect of Initial Concentration
The effect of Hg (II) concentration on its sorption was examined over concentration ranging from 5 ppb to 120 ppb as presented in Figure 4.8. Importantly the other parameters are keptconstant including the contact timeof 5 hours (Refer to Table 3.3).
Figure 4.8: Hg (II) Extracted (%) vs. InitialConcentration (ppb)
It can be observed that the % Hg (II) extracted does not particularly affected by the increase of initial Hg (II) concentration as the extracted Hg (II), maintains about 90%
throughout the concentration change. This is expected since the Hg (II) concentration is measured in ppb, which is small thus concentration ranging from 5-120 ppb does not
have a significant influence on the extraction capacity. However, the variation of initial concentration on the extraction of Cd (II) as can be observed in Figure 4.9. This is obvious since the concentrations are measured in ppm, which is higher.
B
o
S
O 100
90
80
70
60
50
40
30
20
10
10 15 20 25 30 35 40 45 50
Initial Concentration (ppm)
Figure 4.9: Cd (II) Extracted (%) vs. Initial Concentration (ppm)
Figure 4.9 indicates that the removal percentage decreases with the increase in imtial Cd (II) concentration. With the same adsorbent dosage, pH and contact time, there is a drop in removal percentage when initial Cd (II) concentration is high. This is due to the fact that with lower initial Cd (II) concentrations, sufficient adsorption sites are available for the sorption of the metal ions. However, at higher concentrations the number of Cd (II) ions is relatively more as compared to the available adsorption sites. Hence, the percentage removal of Cd (II) depends on the initial Cd (II) concentration and decreases with increase in initial Cd (II) concentrations.
4.4 Effect of Initial Contact Time
Figure 4.10 elaborates that the Hg (II) extracted very rapidly when contacted with the adsorbent. It is observed that after 4 minutes the extraction/adsorption can occur up to 65.1%, 90.83% and 91.75% for 5, 20 and 60 ppb initial Hg (II) concentrations respectively. Moreover, it can also be noted at the initial period of contact time, ranging from 0-40 minutes, the effect of initial concentration is obvious as compared to that after 5 hours of contact time, as can be observed in Figure 4.8. Increase in the initial concentration will result to an increase in the adsorption capacity.
CO X
100
90
15 20 25
Initial Contact Time (min)
30
-•—Hg5ppb -fa—-Hg20 ppb
HgBOppb
35
Figure 4.10: Hg (II) Extracted (%) vs. Initial Contact Time (min)
40
4.5 Effect of pH
pH values ranging from 3-8 are considered to study the effect of pH to towards the adsorption capacity of the adsorbent with other parameters kept constant. It can be noted from Figure 4.11 that higher adsorption % is observed for pH value of 3 and 4.
Referring to Figure 2.1, the pH scale, the lower the pH value the more positively charged and thus more acidic. Even though the Hg (II) is positively charged, the pH measurements are that of the solution or adsorbate and not the adsorbent, and thus attraction phenomena of different charges cannot be considered. However, the positively charged adsorbate solution results in it being more acidic, and as mentioned previously, acid will further open the pores of the adsorbent resulting to an increase in adsorption capacity. Furthermore, even though declining trend is observed, the adsorption is still high with the lowest at pH of 6 still have 80.655% adsorption capability.
Figure 4.11: Hg (II) Extracted (%) vs. pH Value
4.6 Effect of Adsorbent Dosage
Four different adsorbent dosages of 0.05g, O.lg, 0.15g and 2.0g are utilized to study its effect on the adsorption capacity of the adsorbent ('rambai' leave). Figure 4.12 indicates a decline after reaching optimum amount of 015 g adsorbent with 92.54% removal. This decrease in removal percentage with increase in the dosage of the adsorbent can be attributed to the formation of clusters of 'rambai' leaves particles resulting in decreased
surface area.
Figure 4.12: Hg (II) Extracted (%) vs. Adsorbent Dosage (g)
4.7 Reproducibility
The experiments are conducted in 2 trials to investigate the reproducibility of the results obtained. As can be observed in Figure 4.13, it can be concluded that the results obtained are reliable as it is fairly reproducible (Note: One outlier for Hg 5ppb Trial 2 sample can be neglected).
100
_ „ k
*^^:-"***"*~*^———^^__ " __^™___a™_--«i--*^f!!i /
/♦
^
/♦-^ ""
60 | /
so / /
s 50- /// /// —•—Hg 5 ppb Trial 1
11]
40
—E-~Hg 20 ppb Trial1
3 . -*—Hg 60 ppbTrial1
X
30 If ♦ Hg5ppbTrial2
Hg2QppbTrlal2
II
20
10
II Hg60 ppbTrial 2
I
0_
3 5 10 15 20 25
Initial Contact Time (min)
30 35 40
Figure 4.13; Reproducibility Determinatipn Considering 2 Trials
4.8 Adsorption Isotherm
The relationship between the amount of a substanceadsorbed per unit mass of adsorbent at constant temperature and its concentration in the equilibrium solution is called the adsorption isotherm. The Langmuir and Freudlich isotherms are the equations most frequently used to represent the data on adsorption from solution. Langmuir and Freundlich isotherms are represented by the following equations [9]:
Ce/Qe = aLCe/KL+l/KL logQe-logKF + (l/n)logCe
(5) (6)
4.8.1 Langmuir Isotherm
Langmuir model is the simplest theoritical model for monolayer adsorption onto a surface with finite number of identical sites. It is originally developed to represent
chemisorption on a set of distinct, localized adsorption sites. Langmuir has developed a theoritical equilibrium isotherm relating the amount of gas adsorbed on a surface due to the pressure of the gas. The equation is applicable to homogeneous adsorption where adsorptionprocess has equal activationenergy, based on the following assumptions [9]:
i) Molecules are adsorbed at a fixed number of well-defined localized sites ii) Each sites can hold one adsorbate molecule
iii) All sites are energetically equivalent
iv) There is no interaction between molecules adsorbed on neighbouring
sites
The general Langmuir equation is as follow:
Qe=KLCe/(l+aLCe) (7)
When linearized, Eq. (3) becomes Eq. (1). Kl and oil are the equilibrium constants of Langmuir equation. Plotting Ce/Qe against Ce as in Figure 4.14, yields a straight line with slope, <xl/Kl and intercept 1/KL. The ratio a\J Kl indicates the theoritical monolayer saturation capacity, Qe. Figure 4.14 represent the Langmuir Isotherm plot and Table 4.4 illustrate the Langmuir isotherm parameters obtained.
0.30
0.25
0.20 3
|0.15
o U
0.10
0.05
0.00
y = 0.0137x+0.128 R2 = 0.8703
6 Ce(ppb)
Figure 4.14: Langmuir Isotherm Plot
Table 4.4: Langmuir Isotherm Parameters Langmuir
Parameters Value
Slope, oj KL 0.0137
Kl/oil 72.9927
y-intercept, 1/ KL 0.128
KL 7.8370
R2 0.8703
10 12
4.8.2 Freundlich Isotherm
Freundlich expression is an empirical equation applicable to non-ideal sorption on heterogeneous surface as well as multilayer sorption. The model is given as [9],
Qe = KFC I/n (8)
If the concentration of solute in the solution at equilibrium, Ce, is raised to the power of
1/n, with the amount of solute adsorbed being Qe, then Ce1/n/ Qe are constant at a given
temperature. Kf indicates relative indicator of adsorption capacity, while the dimensionless, 1/n, is indicative of the energy or intensity of the reaction and suggests the favoribility and capacity of the adsorbent/adsorbate system. According to the theory, n>l represents favorable adsorption conditions. Eq. (8) is linearized into logarithmic form for data fitting and parameter evaluation to become as Eq. (6). Figure 4.15 represent the Freundlich Isotherm plot.
8 3
-2vQ-
-0=2-
-0,0-
y = 0.8588x+ 0.8367 FT = 0.95
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
logCe
Figure 4.15: Freundlich Isotherm Plot
1.2
Table 4.5: Freundlich Isotherm Parameters
Freundlich
Parameters Value
Slope, 1/n 0.8588
n 1.1644
y-intercept, log Kp 0.8367
KF 6.8670
R2 0.9500
Since the calculated correlation coefficients are consistent and closer to unity for Freundlich Isotherm model than the Langmuir Isotherm, therefore, the adsorption isotherm could well be explained and approximated more favorably by Freundlich
Isotherm model for 'rambai' leaves adsorbent
4.9 Adsorption Kinetics
The prediction of kinetics is necessary for the design of sorption systems. Chemical kinetics explain how fast the rate of chemical reaction occurs and also on the factors affecting the reaction rate. The nature of sorption process depends on physico-chemical characteristics of the adsorbent and system conditions like temperature, pressure, etc.
Measurement of sorption rate constants is an important physico-chemical parameter to evaluate the basic qualities of a good adsorbent such as time required for the adsorbent to remove particularmetals/compounds, efficacy ofthe sorbent, etc. [7].
In batch adsorption processes the adsorbate molecules diffuse into the interior of the porous adsorbent. It was investigated that the adsorption of Hg (II) ions from aqueous solution is a linear phase for 15 minutes (noted that the concentration is in ppb), while the adsorption of Cd (II) ions is a linear phase for 300 minutes (noted that the concentration is in ppm). This behaviour can be attributed to the utilization of available adsorbing sites on the surface of the adsorbent. After this phase, sorption of Hg (II) and Cd (II) was almost ignorable. This might attributedto extremely slow diffusion of metal
ions from the surface film into the micro pores which are the least accessible sites for adsorption. In order to observe the sorption process of Hg (II) ions, on 'rambai' leaves adsorbent, four kinetic models were implemented, which includes, Bangham, pseudo- first-order, pseudo-second-order, and intra-particle diffusion.
4.9.1 Bangham's Equation
The rate constant, Kr for the sorption of Hg (II) were calculated using the simplest form of Bangham Equation [7]:
dQ/dt-Qt/mt (9)
The intergral form can be written as:
Qt = Krt1/m (10)
Assessment of the rate constants is possible by simple linear transformation of the equation:
logQt=logKr + (l/m)logt (11)
As can be observed from Figure 4.16, the linearity obtained plot indicates the applicability of the (l/m)th order kinetics for the system under observation. The adsorption rates constant was calculated from the intercept and slopes of the straight lines and are reported in Table 4.6.
1.47
1.46
1.45
1.44
a 1-43
S 1.42
1.41
1.40
1.39
1.38
y = 0.103x +1.2051 R^ 0.9968
1.7 1.8 1.9 2.1 ! 12.2 2.3 2.4 2.5 2.6
Figure 4.16: Bangham's Model Plot
Table 4.6: Bangham's Model Parameters Bangham's
Parameter
Slope, 1/m
y-intercept, log Kr Rate Constant, Kr (mg/g min) RJ
4.9.2 Pseudo-first-order Model
The pseudo-first-order equation can be written as:
Value
0.103 1.2051
16.036 0.9968
dQt/dt-Kf(Qe-Qt) (12)
Where Qt (mg/g) is the amount of adsorbate adsorbed at time t, Qe (mg/g) the adsorption
capacity inequilibrium, Kf (min"1) is the rate constant for pseudo-first-order model and t
(min) is the time. After definite integration by applying the initial conditions Qt = 0 at t - 0 and Qt - Qt at t = t, the equation becomes:
log(Qe-Qt) = logQe-Kft/2.303 (13)
Adsorption rate constant, Kf and adsorption capacity, Qe for the adsorption of Hg (II) ions by 'rambai' leave adsorbent were calculated from the slope and intercept of the plots of log (Qe - Qt) vs. t as displayed in Figure 4.17 and the parameters are reported in
Table 4.7.
y = -0.0058x +1.2591
^ = 0.9126
250
t (min)
Figure 4.17: Pseudo-first-order Model Plot
Table 4.7: Pseudo-first-order Model Parameters
Pseudo-First-Order Parameter
Slope, Kf/2.303 y-intercept, log Qe Qe(mg/g)
Rate Constant, Kf (min") R2
Values
0.0058 1.2591 18.159 0.0134 0.9126
300
4.9.3 Pseudo-second-order Model
The pseudo-second-order model can be presented in the following form [7]:
dQt/dt = Ks(Qe-Qt)2 (14)
Where, Ks is the rate constant of the Pseudo-second-order model (g/mg min). Definite integration of Equation 14 for boundary conditions Qt = 0 when t = 0 and Qt= Qt at t = t, the following form of equation can be obtained:
t/Qt=l/(KsQe2) + (l/Qe)t (15)
The imtial sorption rate constant, h (mg/g min), at t = 0 can be defined as [7]:
h-KsQe2 (16)
The initial sorption rate, h, the equilibrium adsorption capacity, Qe and the pseudo-order rate constants Ks were obtained from the slope and intercept of the plots of t/Qt vs. t as displayed in Figure 4.18, while its parameters values are reported in Table 4.8.
12
10
8
I
c
E 6 3
i t
g
y = 0.0331X + 0.5207 R* = 0.9993
50 100 150 200 250
t(mln)
Figure 4.18: Pseudo-second-order Model Plot
300
Table 4.8: Pseudo=second-order Model Parameters
Pseudo-Second-
Order Parameter Values
Slope, 1/Qe 0.0331
Qe(mg/g) 30.2115
y-intercept, l/(KsQe2) 0.5207
Rate Constant,
Ks(mg/g min) 0.0021
R2 0.9993
h (mg/g min) 1.9205
350
Since the calculated correlation coefficients are consistent and closer to unity for pseudo-second-order kinetics model than the others, therefore, the adsorption kinetics could well be explained and approximated more favorably by pseudo-second-order
kinetic model for 'rambai' leaves adsorbent.